Add "hidden" doc

doc/man/man5/slapd-config.5

@@ -1216,6 +1216,13 @@ which they are defined.  They are supported by every
 type of backend. All of the Global Database Options may also be
 used here.
 .TP
+.B olcHidden: TRUE | FALSE
+Controls whether the database will be used to answer
+queries. A database that is hidden will never be
+selected to answer any queries, and any suffix configured
+on the database will be ignored in checks for conflicts
+with other databases. By default, olcHidden is off.
+.TP
 .B olcLastMod: TRUE | FALSE
 Controls whether
 .B slapd